The invention relates to a plate for columns performing distillation and/or absorption processes, having through openings in the plate body provided with inlet weir and--in given case--with outlet weir.
There are known many kinds of columns for performing distillation and/or absorption processes. The most widely used of them are the plate columns. The plates are designed to perform an intimate contact between the vapour and/or gas streaming upwards in the column and the liquid descending from the top. Distinction can be made between types of plates with and without drainage. Plates with drainage are the so-called bubble cap plates, valve plates and screen plates depending on their structural elements for contacting both phases (such as bubble caps, valves, holes). Valve plates and screen plates may also be used in combination.
Up-to-date screen plates can be constructed as--in view of the main flow direction of the liquid--simple or multiple stream cross-flow plates or centrifugal plates, wherein there are drilled or pressed circular through holes perpendicular to the plane of the plate body and having diameters of 10 to 15 mm. The holes are distributed at the corners of squares or triangles so that their centres are arranged at distances of 20 to 45 mm.
To form an opinion of up-to-date screen plates (as well as other kinds of column plates) one has to consider several aspects of the process to be carried out, the operation of the column and the costs of the construction. Until now, it is impossible to create a numerical value which would express all these aspects, therefore screen plates can not be characterized by an objective number obtained from measurements, thus, the aspects can be observed only individually, and the factors should then be compared to such values of other plates.
A comparison of the technological aspects can be made in view of the flow speed range (as industrially available) of the vapour and/or gas plotted against the whole cross section of the plate--for a given construction, for a given mixture to be separated and/or for a given combination of gas and liquid, at a constant flow rate of the vapour and/or gas and the liquid (m.sup.3 /h)--in view of the hydraulic resistance (pressure drop) due to the plate when operated; as well as in view of the possible efficiency to be attained in the industrially available flow speed range of the vapour and/or gas. These three factors are not independent of each other, because the pressure drop caused by the plates and the plate efficiency depend on the speed of the vapour and/or gas. Plate efficiency is interpreted as a ratio between the actual separation and the separation which would be obtained if a thermodynamic equilibrium were reached on the plates. (Under separation there is meant a matter separation due to distillation or absorption e.g. separation of a liquid from another liquid by distillation as alcohol from water; separation of a gas component from a gas mixture by absorption so that the gas component is absorbed in a liquid as CO.sub.2 in water.)
As screen plates have only a few structural dimensions having influence on the technology (diameter of the plate, length and height of the outlet weir, diameter of the holes, distribution of the holes, thickness of the plate body), their effect on the flow speed of the gas, on the hydraulic resistance (pressure drop) and on the efficiency may be examined separately. In this respect it has to be noted that the quotient of the plate diameter and the length of the chord-like outlet weir generally equals to 0.6 to 0.7 in the case of simple stream plates and 0.6 to 0.5 for double stream plates. With modern constructions, the height of the outlet weir is max. 20 mm. because a higher weir would only increase the hydraulic resistance (pressure drop) occuring on the plate. This designing principle is based upon the experimentally prooved fact that, at the place of contacting both phases, the transfer of matter between the vapour and/or gas and the liquid takes place mainly spontaneously, therefore it is not necessary to have a long flow path for the vapour and/or gas.
The optimum distribution factor of the holes (a nondimensional quotient of the hole centres and the hole diameter)--as proved by tests (Chemie-Ing. Technik. 34./1962/ No. 4, p. 290)--equals about 2.8.In case of a lower distribution factor, the efficiency decreases due to the fact that the vapour and/or gas streams passing through the holes of relatively large diameter and arranged too close to each other, are fusing into big bubbles, thus, the transfer of matter is adversely influenced. Although with the decrease of the distribution factor of the holes the number of holes in a plate and the sum of their cross sections is increasing, which results in a decrease of the pressure drop, this decrease has in general no influence on the decrease of efficiency mentioned above. At the same time, increasing the distribution factor of the holes higher than its optimum value (2.8) results in an increase of the pressure drop and has an adverse effect on the efficiency, because between holes which are distributed at too large distances there remains a considerable amount of liquid which is not contacted with the primary vapour and/or gas streaming upwards.
When considering operational aspects, there must be three main requirements in sight: the holes must not be stopped, the liquid--in counterflow to the gas and/or vapour streaming upwards--should not run through the holes, and the screen plate should be operated--with unchanged efficiency--in a wide range of the flow speed of the vapour and/or gas i.e. within wide charge limits. An increase of the flow speed involves namely an increase of the production (e.g. a greater amount of the material obtained), and the decrease of the flow speed means a decrease of the chargeing possibility. Due to operational conditions, it may be necessary either to increase or to decrease the production.
Flow-down of the liquid through the holes of the screen plate takes place when the flow rate of the vapour and/or gas is low, and thus, the kinetic energy of the vapour and/or gas streaming upwards is not able to maintain the liquid over the surface of the plate body. Flow-down ceases when the lower limit of the vapour and/or gas load on the screen plate is reached and since then, the efficiency of the screen plate remains constant until the upper limit of the vapour and/or gas load is reached, however, the pressure drop increases continuously.
The most advantageous operational feature with modern screen plates--as already mentioned--resides in that the efficiency is unchanged over a wide range of flow speed of the vapour and/or gas, as well as in the fact that stopping, clogging of the holes does not take place due to their relatively great dimension, however, they have the disadvantage that the pure liquid flowing onto the plate has a greater density than the liquid already containing vapour and/or gas, present on that part of the plate at a distance from the liquid inlet. Due to this fact, flow-down of the liquid of greater density is more intensive near the inlet than at places of the plate situated at a distance therefrom, due to the fact that the hydrostatic pressure of the pure liquid is higher than that of the "foamy" liquid. In order to eliminate this phenomenon, the plate is, according to a known solution, formed with an inclined part on the inlet side, or the whole plate is mounted in the column so that it is inclined towards the main flow direction. These constructional changes are influencing--obviously in adverse sense--the price of the screen plates.
The flow speed range of the vapour and/or gas to be realized with up-to-date screen plates can, due to the sophisticated influences, only be determined experimentally. Due to the fact that there are only few and relatively simple constructional dimensions exerting influences, the modern screen plates can be classed among the cheapest ones, and still they become technologically less advantageous only in the range of the higher flow speeds of the vapour and/or gas as compared to the valve plates or to the valve screen plates which are much more expensive.
The modern screen plate is operationally more advantageous than the plate having mobile valves or the screen plate with valves because the mobile valves are subject to wear and their operational safety is decreased by the risk of seizure and/or hanging up.
There were attempts to decrease the hydraulic resistance (pressure drop) of the screen plates thereby that instead of drilling or pressing holes with sharp edges, protruding necks were drawn upwardly out of the plate body so that the upwardly flowing vapour and/or gas stream was introduced through a rounded inlet into the liquid layer on the plate. Experiments show that thereby the hydraulic resistance (pressure drop) of the screen plate can be decreased by an average value of 30% depending on the rounding-off angle of the inlet.
Observations and measurements prooved that the height of the liquid stream flowing over the screen plate is not uniform but, due to the liquid friction, it decreases from the inlet weir towards the outlet weir. The difference in height is particularly considerable with screen plates with high liquid load: it can be even 15 mm over 1 m of the plate width. This so-called hydraulic gradient has the effect that near to the outlet weir i.e. at the place of the lower liquid level a higher amount of vapour and/or gas flows through the plate than near to the inlet, in other terms: the screen plate does not operate uniformly. This fact is adversely influencing the efficiency of the whole screen plate.
In order to eliminate the hydraulic gradient, according to a known solution, straight slots are split between the rows of holes in the plate body, and then they are widened by pressing. Through these slots the vapour and/or gas is flowing out obliquely to the plane of the plate body towards the outlet and transmits an impulse to the liquid, being sufficient to overcome the liquid friction so that the hydraulic gradient is practicially eliminated. By this relatively simple constructional change, due to which also an eventual backward mixing of the liquid towards the inlet can be prevented. The efficiency of this kind of screen plates having slots is considerably increased but without a meritorious increase of the hydraulic resistance (pressure drop).
As a result of further developments in the field, screen plates were made of expanded sheets, wherein that part of the plate remaining between the inlet and the outlet area is formed of expanded sheet sections. The slots of the adjacent sheet sections are oppositely oriented. The rhomboidal slots of e.g. 42.+-.5 mm should have to direct the vapour and/or gas flowing upwardly therethrough in an oblique direction. As, however, due to the production technology of the expanded sheets, the rhomboidal slots have only a limited obliquity, while their dimension is greater than the hole diameter (10 to 13 mm.) of the modern screen plates, and the thickness of the bars separating the slots is generally small (2 to 3 mm.), only a part of the upwardly streaming vapour and/or gas is forced to flow in an inclined direction. For the same reason, the vapour and/or gas streams flowing only at distances of few millimeters from each other, easily unite to big bubbles (coalescence). This is why the efficiency of the plates made of expanded sheets is lower than that of well constructed screen plates. At the same time, as a result of the high vapour and/or gas speed, the hydraulic resistance of the plates made of expanded sheets is in spite of the big flow area resulting from the big slots, higher--within the efficiency range to be reached at all--than that of the screen plates at the same or at a higher efficiency. The higher energy consumption due to the higher resistance is also disadvantageous.
A further drawback of the plates made of expanded sheets resides in that a flow-down of the liquid through the big slots making a big flow area can not be eliminated except by a vapour and/or gas speed which is higher than required in the case of the modern screen plates. Therefore, such plates made of expanded sheets are particularly sensible to changing load or to load variations.
A further technological drawback resides in that on the one part, that the plates made of expanded sheets may be deformed under effect of heat in the column (and thereby the width of the slots would further be increased), on the other part, their efficiency--under comparable conditions--is much more lower than that of e.g. the valve plates.
It can be stated from what is said above that plates made of expanded sheet does not represent a general progress in view of the procedure to be carried out.
As to the aspects of operation, the slots of the expanded sheet are not subject to clogging as they are big and obliquely directed. Further on, the plates made of expanded sheets are not expensive. In spite of all this, such structures are inferior to the modern screen plates because of their low efficiency and relatively limited range of loadability. Thus, it can be stated that although the slotted screen plates have somewhat increased efficiency over the conventional screen plates but their hydraulic resistance (pressure drop) is not lower than that of the screen plates. On the other hand, the efficiency of the plates made of expanded sheet is necessarily lower than that of the modern screen plates, and also their hydraulic resistance only seems to be more advantageous.
The invention aims at developing a plate construction the efficiency of which, for a wide range of loading, at least reaches or surpasses that of the most modern valve plates or valve screen plates but its pressure drop is lower and besides also the risk of stopping and flow-down is decreased. A further aim of the disclosure resides in that the production costs of the plate construction according to the invention should be lower than the production costs of the valve plates or screen valve plates.
The invention is based upon the following recognition:
On the plates of the distillation or absorption columns the efficiency of the separation process is not specified basically by the resistance of the vapour and/or gas and the liquid (vapour or gas film, and liquid film) against molar diffusion, it rather depends on the intensity of the macroscopic mixing carried out on the plates. According to experiments, as already referred to, the matter transfer between the vapour and/or gas and the liquid (distillation or absorption) takes place mainly momentaneously at the place of contact of both phases, thus, the efficiency of the distillative or absorptive matter transfer depends on how effectively the mixing itself can move away the liquid quantity contacting the vapour and/or gas in order that thereby a free way be left for the liquid not yet contacted with the vapour and/or gas. As known, the mixing on the plates is performed only by the upwards flowing vapour and/or gas streams--through pulse transfer--while the liquid stream conveys only that quantity being fully or partially mixed with this vapour and/or gas.
When considering the above recognition, we met the phenomenon or process known as "mixing model" in the art. It is prooved to be proper also by recent results of the development as set forth below:
As according to the mixing model, the mixing effect on a plate is carried out only by the vapour and/or gas stream, it is convenient to represent the flow direction of the vapour and/or gas on spatial Cartesian coordinates X-Y-Z in order to investigate mixing intensity, wherein X represents the main flow direction of the liquid in the plane of the plate. Y is the coordinate perpendicular thereto and Z is the vertical coordinate.
On modern screen plates the vapour and/or gas flows--except for a possible slight spiral motion--only vertically, thus, it is transmitting an impulse to the liquid only along the axis Z, therefore there is no full turbulence, thus, the efficiency of the mixing of the liquid may not be 100%. This explains why on modern screen plates increasing the height of the outlet weir, and thereby the height of the liquid layer on the plate, over 20 mm does not result in a substantial increase of the plate efficiency.
According to the mixing model, on slotted screen plates the vapour and/or gas is mixing the liquid quantity by a impulse transfer not only in the direction of axis Z but also in the direction of axis X, and this results in a better efficiency. In principle, the oblique vapour and/or gas inlets of the expanded sheets are assumed to lead to the same result. On plates made of expanded sheet sections with slots oriented in different directions, the liquid is mixed in the directions of the axes X and Y, and due to the buoyancy acting on the vapour and/or gas bubbles also in the direction of axis Z. As, however, due to the slight obliquity of the slots, a part of the vapour and/or gas flowing through the slots of the expanded sheet flows also in vertical direction, mixing in the directions X and Y loses in its efficiency and the mixing effect in the direction Z remains dominant. Although the mixing effect is increased as compared to that of the screen plates, this, however, does not result--due to the above-mentioned coalescence--in an increase of the efficiency; because the mixing effect of the larger bubbles is namely not so intensive as that of a greater amount of smaller bubbles. At the same time, the big bubbles developing due to the coalescence are prevented from breaking up (dispersion) as a result of incomplete turbulence on the plate.